Heat treatable aluminum alloys rely on the controlled precipitation of solute alloying elements to achieve desired mechanical properties such as tensile yield strength, ultimate tensile strength and elongation. This is referred to by those skilled in the art as precipitation hardening. It is also recognized by practitioners of the art that hardening phases in the heat treatable alloys include solute clusters or Guinier-Preston (GP) zones, transition precipitates, transition phase particles, and to a lesser degree, equilibrium phase precipitates. [Hatch, John E., ed., Aluminum Properties and Physical Metallurgy, ASM, OH (1984)] With the exception of equilibrium phase precipitates, these hardening phases are not present to a significant degree in as-cast aluminum, and in order to strengthen the alloy, several thermal and/or mechanical treatments are typically employed.
The amounts of soluble alloying elements in heat treatable aluminum alloys exceed the room temperature or near room temperature solid solution solubility limits. Therefore, as-cast heat treatable aluminum alloys typically contain secondary phase particles, which are also known in the metal arts as intermetallic precipitates, equilibrium phase precipitates or simply precipitates. The precipitates found in as-cast alloys are typically coarse and incoherent with the lattice of the aluminum crystals or grains. Further, the as-cast precipitates may exist at grain boundaries. These forms of precipitates do not generally impart significant strength to the aluminum alloy, and may be detrimental to properties such as fatigue and fracture resistance.
A thermal processing step used to strengthen the heat treatable alloys, is called “solution heat treatment” (“SHT”) or solutionizing treatment. The SHT is conducted at an elevated temperature, or solutionizing temperature, at which the alloying elements have maximum solubility in the aluminum solid solution, while avoiding equilibrium melting. When the solution heat treated alloy is cooled, the solid solution becomes supersaturated; the equilibrium solubility of the alloying element in the aluminum solid solution is exceeded. This provides a thermodynamic driving force for the precipitation of the second phase particles.
Precipitation of solute alloying elements is further controlled by the diffusion rate of the solute. Diffusion is a kinetic phenomenon, and the diffusion rate decreases as the temperature decreases. The effect of slowing diffusion rates due to cooling is to decrease the precipitation rate of second phase particles. Therefore, as the alloy is cooled, precipitation is favored by supersaturation of solute, but opposed by slower solute diffusion rates.
To achieve desired mechanical properties of the heat treatable aluminum alloy, it is desirable to maintain the supersaturation of the alloy as it is cooled to ambient room temperature. It is possible to maintain a supersaturated condition at room temperature by cooling the alloy at a rate that is fast enough to minimize diffusion and thus minimize precipitation of solute atoms. Cooling after SHT is referred to in the art as “quenching.” Quenching to room temperature is typically practiced in the trade as: air quenching, where the alloy is cooled in ambient air (either with or without a fan) or water quenching, where the alloy is immersed in water or an aqueous solution or sprayed with water or an aqueous solution.
After quenching, many solution heat-treated alloys will exhibit increases in mechanical properties at room temperature, due to precipitation of hardening phases. This is referred to by practitioners of the art as “natural aging.” Natural aging, to a point where the mechanical properties of the alloy are stable and do not change with time, puts the alloy into what is known as a T4 temper. Controlled precipitation hardening of other alloys requires heating for periods at temperatures above room temperature. This practice is known as “artificial aging.” Artificial aging for an artificial aging time period, where peak strength is obtained and where the mechanical properties do not change with further artificial aging puts the alloy into what is known as T6 temper, which is also known as peak strength temper. For some aluminum-magnesium-silicon alloys, designated 6xxx series (or 6000 series) aluminum alloys, such as but not limited to 6061 and 6063, it is possible to attain specified T6 properties when there is no separate furnace SHT. When these alloys are cooled from an elevated-temperature, mechanical working, or shaping process, and they can be artificially aged to attain T6 properties, the alloys may be designated as being in a T5 temper, although T6 is also considered an appropriate designation, providing the mechanical properties meet T6 specifications.
In the extrusion of heat treatable alloys from the 6xxx series, such as aluminum alloy 6061, it is a known practice to heat or “homogenize” a billet of cast aluminum to change the as-cast microstructure, thus allowing better extrusion performance. The homogenized billet is cooled and then reheated before extruding the billet through a die to obtain a desired extruded form, extrusion, or product. The extrusion is then quenched to room temperature either in air, in water, or by using a water mist. The quench rate or cooling rate of this practice will depend upon the geometry of the extruded form, but for a 0.25-inch thick extruded shape, the air quench rate is about 5-10° F. per second. The process of extruding and exiting a die at a temperature similar to the solutionizing temperature followed by quenching is known in the art as “press quenching.” After quenching, the extrusion may be stretched by 0.5-1% in order to eliminate any thermal stress distortion, which may have occurred during the quenching process. Typically, the extrusion naturally ages for eight hours or longer, during handling within the production facility. After natural aging, the extrusion is heated in a conventional furnace to a typical artificial aging temperature, which is about 350-400° F. for aluminum alloy 6061. The cycle time for artificial aging includes the steps of heating the extrusion to the artificial aging temperature and holding or “soaking” the extrusion at the artificial aging temperature for a predetermined artificial aging time. The cycle times required to reach a peak strength temper, which meets the specifications for 6061-T6, or 6061-T5, is about 6-10 hours. A flowchart that summarizes this current commercial practice is presented in FIG. 3.
From the description of a known extrusion process of heat treatable aluminum alloy 6061 presented above, it is seen that current practice requires long cycle times of heating and cooling the alloy to obtain a product in peak strength temper. Inventory-on-demand type of manufacturing is not amenable to the current process, and an inventory must be warehoused to meet unexpected customer orders. The current practice is also expensive in that it involves significant labor-intensive metal handling between processing steps.
It is clear that a process for manufacturing heat-treated aluminum alloy products in a shorter period of time, which could also be operated in a continuous or semi-continuous fashion, is desirable. Such a process would be beneficial for improving productivity and would provide significant cost savings.
Accordingly, it is a primary object of the current invention to provide a method of manufacture of a heat-treated aluminum alloy product in a shortened period.
It is a further object of the current invention to provide a rapid aging method of manufacturing a heat-treated aluminum alloy product in a peak strength temper, which requires shorter artificial aging time.
It is another object of this invention to provide a semi-continuous or continuous method of manufacturing a peak strength extruded product.
These and other objects and advantages of the present invention will be more fully understood and appreciated with reference to the following description.